1. Field of the Invention
This invention relates to a plasma display panel, and more particularly to a method and apparatus for adjusting a gain for each position of a plasma display panel that is adaptive for improving a brightness uniformity of a picture field.
2. Description of the Related Art
Generally, a plasma display panel (PDP) displays a picture by utilizing a visible light emitted from a phosphorus material when an ultraviolet ray generated by a gas discharge excites the phosphorus material. The PDP has advantages in that it has a thinner thickness and a lighter weight in comparison to the existent cathode ray tube (CRT) and is capable of realizing a high resolution and a large-scale screen.
Referring to FIG. 1 and FIG. 2, a conventional three-electrode, AC surface-discharge PDP includes scan electrodes Y1 to Yn and sustain electrodes Z provided on an upper substrate 10, and address electrodes X1 to Xm provided on a lower substrate 18. Discharge cells 1 of the PDP are provided at intersections among the scan electrodes Y1 to Yn, the sustain electrodes Z and the address electrodes X1 to Xm.
Each of the scan electrodes Y1 to Yn and the sustain electrodes Z includes a transparent electrode 12, and a metal bus electrode 11 having a smaller line width than the transparent electrode 12 and provided at one edge of the transparent electrode 12. The transparent electrode 12 is usually formed from indium-tin-oxide (ITO) on the upper substrate 10. The metal bus electrode 11 is usually formed from a metal on the transparent electrode 12 to thereby reduce a voltage drop caused by the transparent electrode 12 having a high resistance. On the upper substrate 10 provided with the scan electrodes Y1 to Yn and the sustain electrodes Z, an upper dielectric layer 13 and a protective film 14 are disposed. Wall charges generated upon plasma discharge are accumulated onto the upper dielectric layer 13. The protective film 14 protects the electrodes Y1 to Yn and Z from a sputtering generated upon plasma discharge, and enhances an emission efficiency of secondary electrons. This protective film 14 is usually made from magnesium oxide (MgO).
The address electrodes X1 to Xm are formed on a lower substrate 18 in a direction crossing the scan electrodes Y1 to Yn and the sustain electrodes. A lower dielectric layer 17 and barrier ribs 15 are formed on the lower substrate 18. A phosphorous material layer 16 is formed on the surfaces of the lower dielectric layer 17 and the barrier ribs 15. The barrier ribs 15 are formed in parallel to the address electrodes X1 to Xm to physically divide the discharge cells 1, thereby shutting off electrical and optical interferences between the adjacent discharge cells 1. The phosphorous material layer 16 is excited and radiated by an ultraviolet ray generated during the plasma discharge to generate any one of red, green and blue visible light rays.
An inactive mixture gas, such as He+Xe, Ne+Xe or He+Ne+Xe, for a discharge is injected into a discharge space defined between the upper/lower substrates 10 and 18 and the barrier ribs 15.
Such a PDP makes a time-divisional driving of one frame, which is divided into various sub-fields having a different light-emission frequency, so as to express gray levels of a picture. Each sub-field is again divided into a reset period for uniformly causing a discharge, an address period for selecting a discharge cell and a sustain period for realizing the gray levels depending on the discharge frequency. For instance, when it is intended to display a picture of 256 gray levels, a frame interval equal to 1/60 second (i.e. 16.67 msec) is divided into 8 sub-fields. Each of the 8 sub-fields is again divided into an address period and a sustain period. Herein, the reset period and the address period of each sub-field are equal every sub-field, whereas the sustain period and the discharge frequency are increased at a ration of 2n (wherein n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field in proportion to the number of sustaining pulses. As the sustain period is differentiated at each sub-field as mentioned above, gray levels of a picture can be implemented.
FIG. 3 schematically shows a driving circuit for the PDP.
Referring to FIG. 3, the driving circuit for the PDP includes a gain adjuster 32, an error diffuser 33 and a sub-field mapping unit 34 connected between a first inverse gamma adjuster 31A and a data aligner 35, and an average picture level (APL) calculator 36 connected between a second inverse gamma adjuster 31B and a waveform generator 37.
Each of the first and second inverse gamma adjusters 31A and 31B makes an inverse gamma correction of digital video data RGB from an input line 30 to thereby linearly convert brightness according to gray level values of image signals.
The gain adjuster 32 adjusts an effective gain for each of red, green and blue data to thereby compensate for a color temperature.
The error diffuser 33 diffuses a quantized error of the digital video data RGB inputted from the gain adjuster 32 into the adjacent cells to thereby make a fine control of a brightness value. To this end, the error diffuser 33 divides the data into a positive number part and a decimal fraction part and then multiplies the decimal fraction part by a Floyd-Steinberg coefficient.
The sub-field mapping unit 34 maps a data from the error diffuser 33 onto a sub-field pattern stored in advance for each bit and applies the mapped data to the data aligner 35.
The data aligner 35 applies digital video data inputted from the sub-field mapping unit 34 to a data driving circuit of the PDP 38. The data driving circuit is connected to the data electrodes of the PDP 38 to latch a data from the data aligner 35 for each one horizontal line and then apply the latched data to the data electrodes of the PDP 38 for each one horizontal period.
The APL calculator 36 detects an average brightness per frame of digital video data RGB inputted from the second inverse gamma adjuster 31B, that is, an average picture level (APL), and outputs information about the number of sustaining pulses corresponding to the detected APL.
The waveform generator 37 generates a timing control signal in response to the information about the number of sustaining pulses from the APL calculator 36, and applies the timing control signal to a scan driving circuit and a sustain driving circuit (not shown). The scan driving circuit and the sustain driving circuit apply a sustaining pulse to the scan electrodes and the sustain electrodes of the PDP 38 during the sustain period in response to the timing control signal from the waveform generator 38.
Such a PDP has a larger size, such as 40″, 50″, 60″ or the like, than other flat panel displays (FPDs). Thus, since a length of each electrode X, Y and Z of the PDP is large, a voltage drop caused by the large electrode length emerges on the center field and the peripheral field of the PDP at a relatively large difference. Furthermore, since a discharge gas is injected into the interior of the PDP at a lower pressure than the atmospheric pressure, a force applied to the substrates 10 and 18 at the center field where the upper substrate 10 and the lower substrate 18 are supported only by the barrier ribs 15 becomes different from a force applied to the substrates 10 and 18 at the peripheral field where the upper substrate 10 is joined with the lower substrate 18 by a sealant (not shown). This causes the PDP to have a different size of cell 1 at the center field and the peripheral field although having a difference depending upon of a model and a dimension of the PDP. As a result, although the conventional PDP has somewhat difference depending upon a panel size, brightness at the center field of the PDP becomes approximately 20% lower than brightness at the peripheral field thereof in each of the horizontal direction (or x direction) and the vertical direction (or y direction) as shown in FIG. 4.
Such brightness non-uniformity at the center field and the peripheral field can not be substantially overcome by the circuit shown in FIG. 3. For instance, the driving circuit as shown in FIG. 3 can control red, green and blue data differently for each data, but can not control the data differently for each center field and each peripheral field of the PDP. Therefore, the conventional PDP raises another problem in that, as brightness at the center field is heightened, brightness at the peripheral field also becomes higher, thereby still presenting a brightness difference between the center field and the peripheral field and hence causing higher power consumption.